Frequent United Airlines fliers recently received an email reiterating carry-on bag rules. United Airlines has installed boxes at major airports that determine bag sizes and is pushing its employees to increase enforcement of the rules, the Associated Press notes.

Carry-on bag size rules are often laxly enforced by airlines. United Airlines will now review carry-on bags at security checkpoints. Passengers with bags that violate the rules will be returned to ticket counters where they will have to shell out $25 to check bags.

United Airlines contends that the increased focus on carry-on bag size is meant to boost available space in overhead bins. A spokesman for United Airlines denied that the move was meant to boost checked-luggage fee revenue.

Still, checked-luggage fees generated $638 million in revenue for United Airlines last year. A United Airlines executive said that the airline would like to boost that by an additional $700 million over the coming four years.

United Airlines stock jumped more than 3% in Wednesday morning trading.